does anybody know, how to get the saved value from a TrreDropdownField. How can you use "objectForKey"?
Can anybody give an example?

Not exactly sure what you mean by objectForKey but what I think your getting at is how you get the BannerLinkIntern object. Well with a TreedropdownField you should have setup a has_one relation on that page of 'BannerLinkIntern' => 'SiteTree' so to access the object in templates you can use $BannerLinkIntern to access the related page or in the PHP $this->BannerLinkIntern() to get the object.

thats all clear and works fine. But if i choose "home" from the list (for e.g.) and save the date to the db, then the page (backend) is reloaded and the value of the list is: "(Choose or type search)".

What do i have to do to show "Home" as the selected value after saving the data to DB?

Sounds like the 'home' value isn't saving. Have you got a has_one relationship setup correctly between this page and sitetree and ran a /dev/build. You should after building a correct has_one relationship a column in your database 'BannerLinkInternID' if you don't have this column then the treedropdownfield cannot save its value.

I'm a clouts, again I don't know what I'm doing wrong.
Same as above not getting id saved into the has_oneID field in the database.

static $has_one = array(
"LinkTo" => "SiteTree"
);

in getCMSFields_forPopup I'm adding and to and returning

$fields->addFieldToTab( 'Root.Main', new TreeDropdownField("LinkToID", "Link To This Page","SiteTree"));

I'm also trying SimpleTreeDropdownField as well, neither work.
Doing a little homepage captions dataobject, there's only about 3, easier enough to swap out manually. But I like using Silver Stripe to the EXTREME! :P
p.s. using SS 2.4